The purpose of this 160 day feeding study was to determine the effects of unwanted contaminants in technical pentachlorophenol (PCP) in dairy cattle by comparing them to pure PCP and non-exposed animals. Parameter studied included clinical examination, weight gain, hoof growth, hematology and clinical chemistry, gross and histopathology, electron microscopy, immunology, hepatic enzyme induction, and chemical analysis of tissue. Results show that technical PCP is significantly more toxic than pure PCP.